add dialog to choose download destination

[dariocast]

Feature

• Added a dialog on Download button pressed to choose a destination.
• Check that a correct filename has been entered, adding '.mp4' if needed.
• Edit _downloadvideo to accept taken filename as second parameter

Solves issue #2

[dariocast]

When the box opens, you select a destination, and a file. You want to use always the default filename and select only the folder?

[swapnilsparsh]

When the box opens, you select a destination, and a file. You want to use always the default filename and select only the folder?

I want to choose only the folder where i want to download my video, nothing else

[dariocast]

When the box opens, you select a destination, and a file. You want to use always the default filename and select only the folder?

I want to choose only the folder where i want to download my video, nothing else

Fixed, now you choose only directory and then download starts

[swapnilsparsh]

@dariocast Thanks Buddy for solving the issue and if you can solve the other issues as well.