1. Fix progress value not updated
2. Introduce TabStatus object and bind both TabHeaderControl and CommandPalette to it
3. Add support for read-only mode indicator
First step towards #8415:
* Introduce `PaletteItem` and derive from it to provide native support
for tabs and command lines (`ActionPaletteItem` / `TabPaletteItem`,
`CommandLinePaltteItem`)
* Remove business logic behind PaletteItem from palette (aka dispatch
commands and preview tabs externally)