Add a check for offline mode to avoid overwriting complete downloads

[graysky2]

For offline mode, a check should be added to avoid overwriting already downloaded content. Perhaps verify a checksum? Currently, when the user hits "Offline mode>Download VIdeos", I believe the script will simply delete a file with the same name and redownload it blindly. Adding the md5sum (or similar) check would prevent this and save bandwidth for users with limits on their accounts (approx 5.2 G of video content here).

For reference, md5sums of the 34 files:

3da1ca87058d661f7a5054479372683e  b10-1.mov
81e688260de9c957d5710736b560d16e  b10-2.mov
e48bae3ba68d8b0522299393245bc41a  b10-3.mov
f6948a3bc54298e72b22799c46a31a16  b10-4.mov
3da1ca87058d661f7a5054479372683e  b1-1.mov
c086d9ed339d020831bf449fe900db2c  b1-2.mov
1edb09b81b457fc2b1369221e0a58bfa  b1-3.mov
243a27a057fa3cc853de63bfeb3fa071  b1-4.mov
f299e334328dc156a175a5bf023830fd  b2-1.mov
b0a12b41ea4bed98be75203e4656ec61  b2-2.mov
81e688260de9c957d5710736b560d16e  b2-3.mov
f6948a3bc54298e72b22799c46a31a16  b2-4.mov
37872eb043ef64f4bf8b77fd4cd38c66  b3-1.mov
b2bed05732b6fa3acd3c0e5bc3de4955  b3-2.mov
e368b944e63657d471e3c61d6fd3f536  b3-3.mov
4703a4d30ba1f52dc90f5845fe48187c  b4-1.mov
2225fc22dacc5686c232b6f46dfae151  b4-2.mov
83bd5ca0e6a63c58b7d7bda596cf9efe  b4-3.mov
b58e0ef6a4c7e5ba307aa08e2b7229ba  b5-1.mov
2345a7d6a9b2c3f661fef49e3494a65b  b5-2.mov
1e47e12cef1cc08fb585ee388361ae70  b5-3.mov
deec41e765e53bbc7fd3e3052edfc60f  b6-1.mov
236b4af7848782717843d0c804e89b5e  b6-2.mov
f2e4d7705a2896e2163da59a6c6f0dee  b6-3.mov
84d80c0675554ba3803383ff124ed979  b6-4.mov
7f84f3aa270088893acbcf129b333cd6  b7-1.mov
7e4e624f3ee263704268f933b15eee36  b7-2.mov
d5f4ab955c97a6392f79ff4571b4af59  b7-3.mov
fa1df9408264a6a33c3a3c5cfdfbd8b6  b8-1.mov
c26dea13bdc4c0f6bbd333afb9b5f888  b8-2.mov
bfc65a1863d663fc3e35db67d574e5c7  b8-3.mov
b0a12b41ea4bed98be75203e4656ec61  b9-1.mov
7cdbb64268cc3738acce5f41e3cb08ec  b9-2.mov
169d1fe371d1caff01389aea7082c629  b9-3.mov