ide: theme: merge default and legacy

This patch modifies the legacy import strategy. Instead of overwrite the default
value with the found format, it merge them.
ide: theme: fix the legacy theme import

Some keys weren't imported due to a bad group name. This also patch makes sure
to erase all legacy theme artefacts from the settings file by removing both
highlighting and colors groups.
ide: theme: fix default theme

When introducing the theme management, the current line and post window text
formats were modified. This patch restores them to their previous value.
ide: theme: fix transparent color

The Qt::transparent color can't be saved into the settings file. When loading
this color at the next startup, it is black. This patch avoid to set color
background when it is Qt::transparent.