Issue 1022: Add GuiTreeview custom filters

[jgehrig]

Issue #1022: Support Treeview filtering

Adds support for custom filtering of the content displayed by GuiTreeView.

Uses QSettings to add the following ~/.config/nvim-qt/nvim-qt.conf options:

[Treeview]
ShowDirs=true
ShowDrives=true
NoSymLinks=true
ShowHiddenFiles=false
ShowSystemFiles=false

Default values shown. Defaults line up with the current implicit GuiTreeview filter values.

TODO:

• Add documentation in the Wiki
• Add a mechanism to set the options from the shim. Windows uses Registry, difficult to set.
• Unit Tests