Device explorer buttons need to be disabled when command is executing

[josesimoes]

nanoFramework area: Visual Studio extension

Because of the latest improvements in UI responsiveness the toolbar commands is not "locked = irresponsive" anymore when executing a command.
This leads to unwanted behaviours like:

• is possible to hit a second command when the 1st one is still executing
• executing a second command or updating the device availability can overrun a command execution and wrongly updating a command availability after the execution completes

When executing a command and performing tasks that impact on command availability the toolbar should have the relevant command buttons disabled.