Clear progress bar text when starting it (#230)

These operations use steps which generate progress bar bytes copied
information text:
* Partition move or copy using GParted's internal block copy
* EXT2/3/4 partition move or copy
* XFS partition copy
The bytes copied text looks like this (after the copy completes and the
time remaining is no longer included):
    1.00 GiB of 1.00 GiB copied

And these operations use steps which generate progress bar information
without text (because the progress bar data only represents a fraction
complete):
* EXT2/3/4 partition resize
* EXT2/3/4 partition create
* EXT2/3/4 partition format
* EXT2/3/4 partition check
* NTFS partition resize
* NTFS partition copy

In the Applying pending operations dialog, while an operation is being
applied there are 2 progress bars.  The top progress bar displays either
a pulse bar or the progress bar data for the current step.  Additionally
for the relevant steps the progress bar generates the bytes copied text.
This text, when available, is displayed in small grey characters just
above the progress bar itself.

Copy a FAT partition and apply.  Bytes copied text is displayed just
above the top progress bar.  Copy an NTFS partition and apply.  The left
behind bytes copied text from the previous operation is displayed,
instead of nothing.

Restart GParted and copy an NTFS partition and apply.  As intended, this
time there is no bytes copied text displayed just above the top progress
bar.

As there is just a single ProgressBar object, single_progressbar, fix
this by clearing the progress bar text each time it is started.

Closes #230 - Missing progress bar text reset when applying operation

src/ProgressBar.cc

@@ -37,6 +37,7 @@ void ProgressBar::start( double target, ProgressBar_Text text_mode )
 	m_target = target;
 	m_progress = 0.0;
 	m_text_mode = text_mode;
+	m_text.clear();
 	m_timer.start();
 	do_update();
 }