Export images with transparent background

[dl3sdo]

Add option to export .png and .tif(f) images with transparent background.

[dg0yt]

We cannot enforce transparent background, but we can enforce white background.
Looking for feedback: Should this be

• White background

For formats without transparency, drawing the white background would be done implicitly.

[dl3sdo]

That change is indeed an option as making the background transparent is sometimes a unrealizable promise.
OTOH a user might wonder what happens if he deselects the checkmark: will the background be black (BTW: this happens if Mapper would allow transparency for .bmp) as he might not think about transparency?
So I personally have a preference for the current wording but I'm fine with the alternative.

[dg0yt]

Personaly I would prefer to avoid message box.
Changing the option as suggested might reduce the need for information. But at least a status bar notification would still be needed: Uninformed users might wonder why the background is always white in JPEG format.

[dl3sdo]

I assume that the average user of Mapper is a fast learner and notices how and why to avoid this dialog box being shown. I modified the dialog to be an information instead a warning and tell about the consequence (i.e., using a white background). At least a dialog is not so easy to be overseen.

[dl3sdo]

@dg0yt: As proposed by you, I tried to use a status bar indication instead. However, this status bar indication won't be visible as it is (normally) immediately replaced by
main_window->showStatusBarMessage(tr("Exported successfully to %1").arg(path), 4000);
We could change the MainWindow::showStatusBarMessage functionality to put multiple messages in a queue and show one after another (maybe by using the QStatusBar::messageChanged() signal) but that means an extension of how Mapper shows status bar messages.
I thus still prefer the message box to avoid fundamental changes for now.

Another issue is that the 'Transparent background' setting is not saved in the map file - but neither is the 'Save world file' option (or 'Tiles' size in KMZ export).
Maybe because we only save Print related options in the <print> element.
I see three options (for another PR):

1. don't save image related options (i.e., transparent background and world file)
2. also save image related options in the <print> element
3. add a new (e.g., <export>) element

What is your opinion?

[dl3sdo]

Pushing the minimal required set of changes.