kcms/touchpad: Clean up right-click settings for better usability

Original commit message(s)

• kcms/touchpad: Clean up right-click settings for better usability

Always show subtitles for the right-click radio buttons, to avoid confusion about controls jumping around while clicking them.

Remove the conditionally visible pair of radio buttons titled "Middle-click by: Pressing {bottom center, left/right}". An option further up already provides access to the same setting, called "Press both [l&r buttons] simultaneously to middle-click".

On the other side of conditional visibility, remove the checkbox "Middle-click by: Pressing anywhere with three fingers" as the corresponding radio button subtitle already mentions this. Being permanently disabled, it only existed to explain the behavior.

In the "Tapping" section below, move "middle-click by (...)" text of "Two-finger tap" radio buttons from parentheses into subtitles. This lines up visually with the "Right-click by..." radio button group further up.

The cognitive dissonance of not having two "Right-click by..." groups work in the same way (clicking vs. tapping) remains for now.

• kcms/touchpad: In left-handed mode, RMB is in the bottom-left corner

Switch the text of the radio button that's usually titled "Right-click by: Pressing bottom-right corner" accordingly.

• kcms/touchpad: Disable Middle-click via L&R corners if not available

Middle-click by pressing both bottom corners only works if the radio button setting "Right-click by: Pressing bottom-right corner" (a.k.a. clickMethodAreas == true) is enabled.

It does not work if instead, the alternate radio button setting "Right-click by: Pressing anywhere with two fingers" (a.k.a. clickMethodClickfinger == true) is enabled.

In the latter case, disable the checkbox "Left and right buttons: Press both simultaneously to middle-click".

This is still somewhat awkward as a radio button group further down in the page determines the state of a checkbox much higher up. We may at some point redesign this in a more fundamental way. Until then, this commit ensures that at least we don't show an option as working when it's actually ignored.

Also, hide this checkbox entirely if it's not supported by the touchpad at all. Other unsupported settings get hidden too, no reason to treat this one differently.

Reason for the change

The Touchpad KCM is already long and complicated; we don't need to burden users with conditionally morphing and hiding controls.

Test plan

Try out all four combinations of:

• "Left and right buttons: [ ] Press both simultaneously to middle-click"
• "Right-click by: (...)"

Also check out the Before and After looks of "Two-finger tap:" options, which only change in presentation while their behavior stays the same.

Screenshots or screen recordings

Press what to...	Before	After
Right-click: Bottom-right corner, Middle-click: Bottom-center edge
Right-click: Bottom-right corner, Middle-click: Bottom-L&R simultaneously
Right-click: Two fingers anywhere, Middle-click: Three fingers anywhere

Bugs fixed

None that I'm aware of, but this conditionally shown second-level selection stuff was confusing as heck. Someone may well have filed a bug report.