kcms/mouse: follow new HIG guidance for header button & fix translation context

The guidance provided in the KDE HIG for button and menu length was recently updated to provide an explicit length target of three words, with a total width not longer than "Configure Keyboard Shortcuts…".

The header button that opens the Mouse Button configuration page does not conform to this yet. This change updates the label from: "Re-bind Additional Mouse Buttons…" to: "Configure Extra Buttons…" and modifies the title of the resulting page to reflect this. This MR also contains a tiny fix to the translation context of the title of that page.

Justification of changes: (expanded from commit message)

• the word "Mouse" can be removed, as it is implicit in the Mouse kcm and already in the title on the same line. "Mouse" is kept in the title of the binding page.
• "Configure…" is longer, but conventional in kcms for this purpose (see e.g. kcm_style) and allows for further configuration options in the future that are not rebinding of keys; the existing "Re-bind" is also rather technical and may not be transparent to all users
• both are commonly used, but "Extra" is shorter and more frequent. This was evaluated with google searches for '"mouse with (additional|extra) buttons"', 'mouse "(additional|extra) buttons)"', '"configure (additional|extra) mouse buttons"', and '"(additional|extra) mouse buttons"'. The variant with extra is always at least roughly twice as frequent as that with additional, and for the final case the ratio is 12:1.

As a small additional fix, the translation context of the title of the mouse binding page incorrectly claims to be a button. This is changed to be a title.

Screenshots before:

Screenshots after: