Add comment functionality to the SQL console

Thanks @nicolas-f for the tips.
Add uncomment functionality to the SQL console
Combine comment+uncomment into one method
Add comment functionality to Beanshell console
Add block (un) comment to Beanshell/SQL consoles
Check if sel==null first when block commenting
Merge pull request #579 from agouge/comment-564

Here it is! Now we can finally comment and uncomment text in the Beanshell and SQL consoles without having to type `--` or `//` for every single line.

I implemented the same commenting behavior I found in other major programs such as IntelliJ, Netbeans and Eclipse. In particular, toggling the comment function gives the following behavior:
```mysql
----CREATE TABLE ...                         --CREATE TABLE ...
--CREATE TABLE ...      <- becomes ->      CREATE TABLE ...
----CREATE TABLE ...                         --CREATE TABLE ...
```

#### Keyboard Shortcuts
* <kbd>ALT</kbd>+<kbd>c</kbd> - **Comment/Uncomment**. I.e., prepend a `--` (SQL) or a `//` (Beanshell) at the beginning of each line containing selected characters (or at the beginning of the current line if the selection is emtpy), or remove them if every line in the selection is commented.
* <kbd>ALT</kbd>+<kbd>SHIFT</kbd>+<kbd>c</kbd> - **Block comment/uncomment**. I.e., surround the selection with `/*` and `*/` (both SQL and Beanshell).

Fixes #564.