I would appreciate very much if someone could recommend me a good text editor for VHDL.
The absolute easiest way to go would be gedit with plugins (specifically, the code snippet plugin)
sudo apt-get install gedit gedit-plugins
gedit has the benefit of working well with GHDL.
Emacs (which should come pre-installed on your machine) has a very useful VHDL mode documented here.
If you want something more full-featured, Scriptum is very full-featured and comes with support for other languages such as Verilog, C/C++, and Xilinx UCF. Its download page is here. (You have to fill out a form, but it's free and doesn't require a license.)
You're opening pandora's box here! Most editors will support syntax coloring for a wide range of languages, including VHDL. So your question could be re-interpreted as a question for just a nice editor.
Vim includes a syntax file for VHDL by default.
Although a little tough to understand at first once you get used to it there are few editors faster or more useful than VIM.
You can install it by running
sudo apt-get install vim-nox or if you want it with the small gtk shell
sudo apt-get install vim-gtk.
Be sure to run
vimtutor if this is your first time using
vim, it will help you get up to speed on how vim is used. Alternativly you could take a look at this online vim tutorial.
My personal choice is Sublime Text. You will have to add a package for Verilog and VHDL but it is very easy and Sublime Text includes modern features such as multicursor.