[lmi] First screenshot tool screenshots

[Vadim Zeitlin]

 Hello,

 I have a first working version of XRCShot tool which is capable of
generating screenshots of the specified panel defined in an XRC file
annotated with the help texts for its children.

 It already looks not too bad, and so far I spent less than 3 hours on it
only. It's far from being finished however, there are at least the
following 2 big problems I'm aware of:

1. Radio box items help texts are not handled: this will need some changes
   to the code as currently I iterate over all windows and radio box items
   are not windows, but this won't be difficult to do (at the price of
   having to explicitly test for wxRadioBox using dynamic_cast).

2. The size of lower HTML window is guessed rather than computed, i.e. it
   could easily become too small for the text shown in it. This is a
   somewhat serious problem as it means I'd have to change my approach of
   showing wxHtmlWindow on screen as I do now and render HTML off screen,
   but -- thanks to Vaclav's help with this -- this should be easy to do as
   well.


 There are also some missing features, please let me know which of those
would be useful (and in which order):

3. Currently only wxPanels are supported, and not wxDialogs or wxFrames.

4. You need to specify the name of the panel to load from the XRC file as
   shown above. The tool could also iterate over all panels but it's not
   clear what should be used for the output file name and the title in the
   screenshot in this case (currently both can be specified on command
   line).

5. HTML is currently hardcoded but could be made configurable by using a
   template in an external file.

6. Output format is always PNG but we could allow selecting among any of
   formats supported by wx.


 Thanks in advance!
VZ