Installer: Remove the need for xterm

The following executes an external process and displays its output in a Qt widget.

```
#!/usr/bin/env python3.7

# Based on https://stackoverflow.com/a/41751956

import sys

from PyQt5.QtCore import pyqtSignal, pyqtSlot, QProcess, QTextCodec
from PyQt5.QtGui import QTextCursor
from PyQt5.QtWidgets import QApplication, QPlainTextEdit

class ProcessOutputReader(QProcess):
produce_output = pyqtSignal(str)

def __init__(self, parent=None):
super().__init__(parent=parent)
self.setProcessChannelMode(QProcess.MergedChannels)
codec = QTextCodec.codecForLocale()
self._decoder_stdout = codec.makeDecoder()
self.readyReadStandardOutput.connect(self._ready_read_standard_output)

@pyqtSlot()
def _ready_read_standard_output(self):
raw_bytes = self.readAllStandardOutput()
text = self._decoder_stdout.toUnicode(raw_bytes)
self.produce_output.emit(text)

class MyConsole(QPlainTextEdit):

def __init__(self, parent=None):
super().__init__(parent=parent)

self.setReadOnly(True)
self.setMaximumBlockCount(10000) # limit console to 10000 lines

self._cursor_output = self.textCursor()

@pyqtSlot(str)
def append_output(self, text):
self._cursor_output.insertText(text)
self.scroll_to_last_line()

def scroll_to_last_line(self):
cursor = self.textCursor()
cursor.movePosition(QTextCursor.End)
cursor.movePosition(QTextCursor.Up if cursor.atBlockStart() else
QTextCursor.StartOfLine)
self.setTextCursor(cursor)

app = QApplication(sys.argv)

reader = ProcessOutputReader()
console = MyConsole()
reader.produce_output.connect(console.append_output)
reader.start('ping', ['heise.de'])

console.show()
app.exec_()
```