Splitters dragging can be broken if you release the mouse outside of the parent window

[nchevobbe]

From https://bugzilla.mozilla.org/show_bug.cgi?id=1327984 (we should mark this bug as resolved when this issue and #83 lands in mc)

Another issue I found when testing this:

1. Open DevTools and select the Inspector panel
2. Start changing width of the Sidebar (using vertical splitter)
3. Move mouse over to the content (page) document area
4. Release the mouse button
5. Move mouse back to the Inspector panel -> the dragging is still in progress even if the mouse button isn't pressed -> BUG

[nchevobbe]

We had to deal with such bug in the inspector when dragging node, if the mouse was released over the content window. The fix, if I remember correctly was to have to mousemove event declared on the window, which would track and capture mouse event even if the mouse is out of the firefox window (or the content window).

[wldcordeiro]

Optionally @nchevobbe we could listen for mouseout I believe and end dragging/reset.