Add expand/collapse feature to the RTags buffer for print-dependencies result

[fperies]

Add expand/collapse feature to the RTags buffer for print-dependencies result.

It would be very convenient to expand and collapse the result of the rtags-print-dependencies command (in the include tree part) so we can easily see all the includes at a same given level. For now and for large code base, it is not very easy to see in a concise way, the include path to an given include file from a source file.

For instance, usage of the TAB key would expand/collapse all the include files at current level as it is done for buffer rtags-reference-tree-mode.

Thanks

[fperies]

Well, I've just discovered the rtags-dependency-tree command which supports this expand/collapse in a very nice way. So I think this issue can be closed.