UI: Add games directory and allow switching of games via controller in popup-menu

[TrentSeed]

At the moment, switching between games in Xemu typically requires using a mouse/keyboard. When using a game controller, it's possible to open the popup menu, and select "Load Disc...", however that launches a native file browser which does not have controller input support, requiring use of a mouse/keyboard to select the target file. The goal of this PR is to make switching between games a bit more convenient and possible using a controller.

This PR introduces a new setting called "Games directory" within Settings > General > Miscellaneous. By default there is no value specified, and when clicked the user is presented with a file browser to select the desired directory containing .iso game files.

When the popup menu is activated (via right click, or start/back from an Xbox Controller S), there is now a "Games" submenu button that when selected, will present the user with a sorted list of games based on the configured directory. Selecting a game will result in Xemu loading the corresponding disc file. In the event no .iso files are detected in the directory, or if the directory is still undefined, there is an option "No games found" that when selected will open the main settings UI so that the user may adjust the games directory as needed.

[mborgerson]

Thanks for addressing the review feedback. I've rebased onto the latest version of the master branch. Can you please fix the build issues, then I'll happily merge.