Dialog styling fixes

I found the following issues with some recent changes of @dkazakov:

DlgLoadWarnings:

• renders the content non-window-modally, off-center, and with a default size of 0x0.
• doesn't match the format of a QMessageBox because of a double icon override

Display Settings pane:

• The OpenGL warnings are inconsistently styled wrt. the Microsoft Store pane

And, with more older code, the Export dialog shows the same issues as DlgLoadWarnings.

To fix it, I standardized DlgLoadWarnings and the Export dialog's minimum size, fixed the double icon, and made sure the former's QTextBrowser isn't leaked if there are no warnings. For the Display Settings pane, I styled the warnings label into a group box, added the correct icon, and corrected the logic as needed.

I have not added any translatable strings, so this could be cherry picked to 5.1.

Before	After

Test Plan

Build Krita.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.