Add ability to remove disconnected copter from table about clever-show

[artem30801]

Added option for removing selected copters from table (in alpha branch)

I think auto-hiding disconnected copters could be unwanted in case of unstable connection (or any issues that leading to connection reset)

Reopen in case of other suggestions on this topic or any related issues.

[goldarte]

Needs to be tested first

[goldarte]

And then merged to master branch

[goldarte]

Tested, works. But when we turn off the power from Raspberry, server thinks that there is still a connection. How can we solve this?

[goldarte]

Also, when I try to remove copters from table manually with <remove_disconnected = True> parameter in server_config.ini, I got this error, and not all copters are removed:
Traceback (most recent call last):
File "server_qt.py", line 251, in remove_selected
row_num = self.model.data_contents.index(copter)
ValueError: <copter_table_models.StatedCopterData object at 0x7f730c020f98> is not in list

[goldarte]

Fixed here: 576e1a5

Needs to be tested again.

[goldarte]

Done here: #55
Tested, works.