Consistent dialog buttons #2498
Description
Currently, novelWriter uses standard buttons as provided by Qt whenever possible. Qt ensures that the style of these buttons follow the style of the OS the app is running on. However, novelWriter has a bunch of non-standard buttons that don't follow such styles. I think it would be better and more consistent if they were all custom and had the same icon themes as the rest of the app.
The placement of standard buttons can still follow the OS. This standardisation feature is separate from the button style, and easier to maintain since it is based on button categories. For instance, Debian Linux and Windows have opposite order of Yes/No buttons, and some platforms place Apply style buttons on the left of dialogs instead of on the right. These adaptations should be kept.