Remove icons before the extensions

Removed due to the following reasons:
- You've to pass the extension specific media path and the image filename
- Txi can install multiple extensions which might have different icons. So you've to assign an icon for every extension of the installation_queue.
- It would take too much effort to implement this feature and I'd rather keep it KISS.
Extension table markup improvements
Get $installType in _applyFixes() and not in _installExtensions()
Rows markup of txi-table
Components uninstallation implemented and improved rendering rows of uninstallation table
Detect extension which is already uninstalled by Joomla itself (due to its xml file). Then only uninstall the remaining extensions from the $uninstallation_queue
Rename 'name' to 'element' in uninstall.php
Add 'plg_' prefix for the plugins in the rendering of the extensions table, because it looks better
Add link to Twentronix Installer repo to the description, which is showed in the output