[Feature] Add menu entry to "install" third party AppImages

[lfom]

Hello!

Thank you for developing and maintaning AppImagePool! I was looking for an app that would simplify the proccess of integrating downloadd AppImages (I know about AppImageLauncher, but I do not want to have an background daemon running all the time just for this) and found an issue here that showed me the right direction.

Right now, it is not clear how AppImagePool "integrates" downloaded apps with the desktop enviroment. After reading the project page and installing the app I could not figure out how it works... Reading #81 I realized that the user must first copy the file to the default location ( ~/Applications) and the file will appear in the Installed tab.

What I suggest would be adding an item to the menu, that now has only Preferences and About the App, that would open a file picker so the user could choose the AppImage file, that would be then copied to the default location and automatically integrated to the desktop environment (i.e. created a .desktop file and icon), something like Install AppImage File. This way the user would be able to integrate any downloaded AppImage to AppImagePool and the system.

Regards

[prateekmedia]

Great Idea, I will try to add the feature this month.