ide: introducing themes management

This patch adds IDE themes management. It has the same behaviour as QtCreator.
There is some default theme provided and the user can copy them to create a new
one. Only user created theme can be edited. For now only default theme is
provided: "default".
ide: themes: set transparent as background default value
ide: themes: disable unused options display for currentLine

Only background color is used for current line, this patch disables so others
options for this entry to improve understanding for users.
ide: themes: remove General and Syntax categories

There is no difference between General and Syntax categories. This patch
consequently removes them.
ide: theme: fix theme formats filling

The bug was introduced by the following patch:
  ide: themes: remove General and Syntax categories
ide: theme: support for legacy theme

This patch adds the ability to the theme management to look for a previous
custom theme and if available import it into a new theme called "My old theme".
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.
ide: theme: set Post Window Text background to transparent

To match the pre-theme-management appearance, this patch sets the Post Window
Text background default value to transparent.
ide: theme: do not set background when the color is transparent

Since the transparent color can't be stored, this patch makes sure that when the
picked brush is Qt::NoBrush (~ color transparent), the background is not set.

It completes the following commit:
  ide: theme: fix transparent color
Merge pull request #1150 from vdonnefort/ide-theme-management

ide: introducing themes management