Respect system color scheme for rectangular selection
As a follow-up to 4e2f90a30b66, in addition to using the system's color
for the text labels, we now also support user-chosen colors for the
label's background and border. The highlight color for the selection
handles and the magnifier is improved, too.
The Use light background setting is now strictly about the
background colour and won't affect the selection handles anymore.
Note: Due to the opacity, we have to use systemPalette.light instead
of systemPalette.window, otherwise for light schemes the contrast
would be too low.
For all color schemes, start spectacle -r and observe the help text in
the center and on the bottom. Look at the selection handles and press
⇧ to show the magnifier's crosshair. Everything should respect
the chosen color scheme and have enough contrast.
Reviewers: Spectacle, broulik, ngraham
Reviewed By: Spectacle, broulik, ngraham
Subscribers: ngraham, broulik, abalaji
Differential Revision: https://phabricator.kde.org/D12657