Fixes issue 4
Rebased off of host master to reduce the chance of a merge conflict
* rdpsign.vb: updated to dynamically determine the path to rdpsign.exe
* rdpsign.vb: updated to show an error if rdpsign.exe cannot be found (now imports system.windows.forms for MessageBox)
* Create Client Connection window: Made some visual + text changes
* Create Client Connection window: Disable the signing section if rdpsign.exe not detected or if no certificates are found
* Create Client Connection window: Change "Disabled" checkbox text to "Sign RDP file" and reverse its logic (so that checked = sign rdp file)
* Create Client Connection window: "Create Signed and Unsigned" checkbox is disabled + unticked if generating an MSI
* Create Client Connection window: Certificate ComboBox defaults to the first certificate rather than blank
* Create Client Connection window: Updated the save/reset settings to include the new signing section
* Create Client Connection window: Updated the Tab-Order
