Have special keyboard controls in selection mode
Prior to this change keyboard controls and behaviour of Dolphin's main view were identical no matter if selection mode was enabled or not. While selection mode makes it impossible to accidentally clear the selection by singular mouse clicks, any press of an arrow key on the keyboard would still clear the full selection which goes against selection mode's objective.
Furthermore, keyboard-only users had no reason to ever enable selection mode because it made no difference to them.
This commit changes this by offering a changed control scheme for key presses while in selection mode. Arrow key presses without modifier now only move focus between items but do no longer clear or change the selection. Similarly, Page Up/Down, Home, and End key presses only move keyboard focus. Enter, Return, and Space key presses now only toggle the selection for the current item.
The type-ahead feature is also changed in selection mode to only move keyboard focus without changing the selection.
This way keyboard users are less likely to clear their selection by mistake. Regression tests are added to make sure this behaviour is kept.
The code changes to change this keyboard behaviour are quite minimal. Most of the added code is for making selection mode accessible. That's because we need to make sure the changed control scheme is properly announced and communicated or a blind user will be left utterly confused why the normal keyboard controls "stopped working".
Enabling or disabling selection mode is announced to accessibility software. Furthermore whenever focus goes to the main view, the selection mode state is also mentioned when active.
@nclarius, @genericity, you both reported this feature request, so if you have comments, please let me know. :) Though I am mostly working on this now because the accessibility quick scan provided by HAN University of Applied Science for my NLnet project reported that it can be confusing that the selection mode does not seem to help in selecting while using a keyboard.