Improve behavior when changing folders in settings dialog

If the background cycling option is enabled, and the user selects a new folder, nothing will be selected in the icon view, but sometimes users will think that is enough to change the folder where xfdesktop will find backgrounds to cylce through, and close the window. Unfortunately, that doesn't actually change the folder; only selecting a background in the new folder will do that.

Selecting one automatically (when background cycling is enabled) is undesirable because users will sometimes navigate around their folder structure to explore what's available, and will sometimes decide not to change the background.

This commit adds an "Apply" button next to the folder selection button when the folder is changed and the background cycling option is enabled. The Apply button is hidden after it is activated (which selects the first background in the icon view).

Other options considered:

• Add a banner on the top warning the user that the folder is not actually selected. But we already have a banner there in multi-monitor setups, and adding extra text for the user to read (especially for users who don't forget about this issue) is annoying.
• Add a confirmation dialog when closing the settings window if no background is selected. Two issues here: 1) we cannot actually do this when the settings dialog is embedded in the Settings Manager, and 2) this would be an annoyance for people who navigate around and then change their mind and want to close the dialog without changing anything.
• Add an extra "Select Folder" button to the dialog's action area that selects the first item in the folder and then closes the dialog (in addition to the "Close" button which would discard changes). I like this option, but unfortunately this isn't possible when the settings dialog is embedded in the Settings Manager.

Closes #212 (closed)