[FEATURE] allow installing plugins from local ZIP packages

[alexbruy]

This may be useful for users who have plugin packages, so they don't need to extract them manually.

[DelazJ]

@alexbruy does it mean that, e.g. on macos, you no longer need to unhide the .qgis/ folder before adding a new plugin picked on its own dev repository ?
Would be great!!

[luipir]

@DelazJ depends how it works:
afe85ca#diff-9d900c8c7853adc2cc0dda54bb27c9d2R553
on osx if plugin path is hide

[borysiasty]

LGTM.

In Essen we can think about moving the action from the main menu to the manager window. But now I can't see any obvious place (except a new empty tab with a small button in the center ;) )

@luipir - I can't test it but I assume it works like in Linux. Hiding/unhiding directories only affects the os GUIs, doesn't it?

[alexbruy]

@DelazJ, yes it works in the same way as Plugin Manager, but for local files. So you don't need to do anything with your ~/.qgis3 directory