Fix rendering of on-screen help texts for fractional scaling

Summary:
In the review for 4e2f90a3 it was suggested to switch from `Text` to
`Label` for the help texts of the rectangular selection screen. While
this allowed to respect the system's colour scheme, there are two visual
regressions compared to before that patch, in particular when fractional
display scaling is active:

With Qt 5.9 and earlier, the text becomes blurry. With Qt 5.11, the text
gets an incorrect colour and is placed well outside of the containing
box, making it totally unreadable.

Both problems could be solved by switching to Qt Quick Controls 2
(introduced in Qt 5.7), where `Label` works better for fractional
scaling. However, Spectacle depends only on Qt 5.6, and we are already
past the dependency freeze.

Therefore an alternative fix is applied: We go back to using `Text`, and
set the text colour manually instead.

Test Plan:
`QT_SCALE_FACTOR=1.5 spectacle -r` should show sharp text at the right
position, with colours adapted to the active colour scheme (e.g. Breeze
Dark). Tested with Qt 5.6 and 5.11.

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D14254