feat(filesystem_browser): various feature additions and fixes.

[kenmcgaugh]

FSDirectoryTreeBody.qml

• Replaced '...' with animated busy indicator.
• Eliminated some benign error messages.
• Prevented pressing "Full Scan" button from re-centring the direcotry list's scroll position to be consistent with just selecting the directory.

FSMainHeader.qml

• Added "Expand Sequences" toggle. When checked will disable the use of fileseq to compress frames into sequences.

FSTreeView.qml

• Added "name_and_count" property to FSDirectoryGroup item to avoid error message.

FileSystemBrowser.qml

• Added callback to automatically stop active scanning when the browser becomes hidden.
• Fixed bug preventing the scanning feedback console from working.
• Fixed copy-paste error where the fileTreeView was incorrectly calling the fileListView.getItemAtIndex().
• Modifed the right-click pop-up to select the item under the mouse if it isn't already selected to avoid confusion over which items the menus acted on.
• Modified the thumbnail tooltip to pass through any mouse events.

config.json

• Added more supported extensions.
• Exposed "non_sequence_extensions" so it can be customised.
• Exposed "follow_symlinks" so it can be customised.

filesystem_browser.py

• Removed unused imports and member variables.
• Added support for the "Expand Sequences" toggle.
• Added support for customising the "non_sequence_extensions".
• Added support for customising the "follow_symlinks".
• Commented out the scanning for subdirs-with-subdirs as it was causing the directory tree to hang for minutes at a time.

scanner.py

• Removed unused imports and member variables.
• Added support for the "Expand Sequences" toggle.
• Added support for customising the "non_sequence_extensions".
• Added support for customising the "follow_symlinks".
• Cleared out some dead code.

[tedwaine]

This is great. Thank you, Ken.