New function: goto line

[Slesa]

have two problems:

• the line number is not calculated correctly when using head lines
• the input dialog should get current line and maximum line

Cursor movement seems to be rocket science in Qt5.

[cloose]

• "line: " ==> "Line:"
• ui->plainTextEdit->document()->blockCount() ==> ui->plainTextEdit->document()->lineCount()
• maybe introduce local constants to make this more readable:

void MainWindow::editGotoLine()
{
    const int STEP = 1;
    const int MIN_VALUE = 1;
    ...

[Slesa]

One thing is left: determine the current line...

[cloose]

The following code should do the trick:

int currentLine = ui->plainTextEdit->textCursor().blockNumber()+1;