Improve UX when loading ROMs from a temporary/non-writable path

[ahigerd]

This commit introduced a warning that is displayed when loading a ROM from a temporary directory, which could happen if double-clicking a .gba file contained inside a .zip file. We also have the existing warning that is displayed when loading a ROM from a non-writable directory (e.g. Program Files due to UAC).

In both of these cases, the user is informed that they may lose save data, but mGBA doesn't offer any mechanism to improve on the situation; the user must understand the problem and manually move the ROM to a better place.

Instead, mGBA could instruct the user to copy the ROM to a better location and provide a file dialog to select a more appropriate path, performing the copy operation automatically. The chosen path should be revalidated for suitability.

This will not only make the solution easier for the user, it is much less likely that the user will click through the warning without paying attention. A user that really doesn't want to do it can still click the Cancel button in the file dialog.

Polish ideas to consider:

• If there are any folders already added to the library, open the file dialog to one of the paths there by default.
• Store the chosen path in a setting so that it can be recalled next time.
• Maybe if the user has already configured the library, the game or the selected folder should be automatically added to it.
• A link to a FAQ might be a good idea.