RDP: Fix resolution scaling and initial resolution settings

Determine initial remote desktop resolution once from settings.

Only use custom width/height when that resolution mode is selected. For current window size, just measure the window.

Handle scaling better.  React to parent window resizes.

Fixes mouse events at incorrect locations (because now the rdpview size always matches the displayed image).

After all this, we get to simplify sizeHint() and paintEvent() code.

Fixes !69

!68 may need a rework if it is still valid.

(cherry picked from commit 5928fed4)