frontend: Fix warning for themeWatcher

[Warchamp7]

Description

Fixes a Qt warning during startup when the AutoReload setting for themes is enabled.

setTheme calls themeWatcher->removePaths and when the theme is first set up themeWatcher->files() is empty.

Any other time except when the theme is first being set up this call will be fine, so the setup of themeWatcher is moved after the first setTheme call.

Motivation and Context

Warnings are bad. Found using QT_FATAL_WARNINGS

How Has This Been Tested?

Built and launched OBS, no more warning.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.