gdbplugin: fix that couldn't reset start debug button icon

Rootcause: icons of starting debug and continue were different and button icon was set to same as continue action when program ends

Solution: now icons of starting debug and continue are same and the button icon is the same as it's default action

before:

the button icon has not been recovered even thought I finish debugging.

after:

now the button uses the same icon even before debugging has started.