Remove unnessary mouse grab.

[karliss]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've updated the documentation with the relevant information (if needed)

Detailed description

• Causes warning spam with Qt wayland backend
• Can sometimes cause crash on wayland
• According to docs shouldn't be necesarry

At least today crash was more likely to happen after following steps:

• open graph widget
• open settings
• close settings with escape
• immediately after closing settings (almost at the same time) start dragging with mouse in the graph widget -> crash

Test plan (required)

Testing process:

• test that dragging view with left click and middle mouse button works in graph widget
• test that dragging mouse across color selection widget within theme editor triggers an update
• try to do the previously described steps for triggering a crash

Test platforms:

• CI Appimage build on Linux with desktop in X11 mode
• CI Appimage build on Linux with desktop in wayland mode
• CI windows build
• CI macOS build
• QT5 build running with wayland backend
• QT5 build running on wayland with QT_QPA_PLATFORM=xcb
• QT5 build with desktop in X11 mode
• QT6 build running with wayland backend
• QT6 build running on wayland with QT_QPA_PLATFORM=xcb
• QT6 build with desktop in X11 mode

Do not merge before all plaforms are fully tested !

Closing issues