[app] Add basic touch support for the remote input/mousepad plugin

Currently the remote input part of the app only really supports mouse input for moving the cursor around, on my PinePhone running postmarketOS 23.12 Phosh (Wayland) and KDE Connect 23.08.4 at least using just a single finger doesn't work, using two fingers works but is unreliable (sometimes it switches to holding left click down instead of just moving the cursor as I move my fingers around), and clicking requires using the dedicated buttons.

So, this is an attempt at adding touch support, in particular moving the cursor with a single finger, tapping with one/two/three fingers for left/right/middle click, tapping and holding for dragging things around as well as two finger scrolling.

Having settings to adjust the sensitivity, scrolling direction and pointer acceleration would be ideal, but at the moment that is outside my current abilities so I probably won't get to that anytime soon.

I've only really tested this on the aforementioned PinePhone by patching 23.08.4 (so Qt5 still), although as far as I can tell the stuff I'm using here hasn't changed between Qt5 and Qt6, and the app still works with the patch on my non-touchscreen laptop with Plasma 6 and KDE Connect from git master.