Implemented basic horizontal **split**
Managed to get proper behaviour for closing horizontal/vertical split views
minor
hbox > vbox order feels more natural
Fixed `CodeArea.setHeight` and `CodeArea.setFontSize` issues w.r.t. split views
Need *always* to apply `Ext.fly ('page-wrap').setDisplayed (false)` due to split views
minor
The `tab-manager` should have the `focused` property set (or cleared)
minor
Ensure that correct `tab-manager` is chosen
Linked `CodeMirro` instances together
Refactored `split_{horizontal,vertical}`
Refactored `beforeclose` from `text-editor` and `image-viewer`
minor fix
Fixed keyboard binding issues
Fix toolbar with "overflown" `button`s (which become then `menuitem`s) by using common type ancestor `component`