Side effect from #84 fix

[nexus226]

The fix implemented in #84 introduced a side effect: now multiple file pickers can be opened for the same and other action, which gets really confusing since they're all titled the same (Load or Save as)

The ideal solution, if possible, would be to limit the amount of file pickers that can be opened per-action or in general to 1 (limit it in general is preferable but it really depends on user preference, maybe make it a toggleable option?), label each window according to their function - e.g. open a file, load/save a preset - and keep the window always on top

[valadaptive]

This has been partially fixed in v0.9.0 (Windows and Mac work properly, but Linux still lets you interact with the main window while a file picker is open).